The morning of Sept. 11, 2001 started off like any other. The Twin Towers stood tall in the Financial District, as they had for more than 30 years.

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At 8:46 a.m., American Flight 11 crashed into the North Tower of the World Trade Center. At first, newscasters weren't sure if it was an accident or a deliberate attack.

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At 9:03 a.m., United Flight 175 crashed into the South Tower, leaving no doubt that this was an attack. Some news channels captured the traumatic moment on live television.

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The second plane exploded upon impact, caused by the ignition of fuel. Now both buildings were burning.

AP Photo/Chao Soi Cheong

People stared from the windows of the Towers, trapped by smoke and flames and destroyed staircases.

REUTERS/Jeff Christensen

This iconic photo captured a man falling from the North Tower. At least 200 people fell or jumped from the Towers.

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At 9:40 a.m., American Flight 77 crashed into the Pentagon. Five minutes later, for the first time in history, the FAA ordered all aircraft to land at the nearest airport.

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At 9:59 a.m. the South Tower collapsed.

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People fleeing the scene by foot were covered in dust and ash.

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At 10:03 a.m. hijacked flight United Flight 93 crashed into a field in Shanksville, Pa. The plane's target was believed to be the U.S. Capitol building. The passengers on board tried to gain control of the flight and divert the hijackers after learning of the other attacks.

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At 10:28 a.m. the North Tower collapsed. It took only 12 seconds for the Towers to fall.

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2,753 people were killed in the New York attack.

An aerial view of the World Trade Center disaster site seen on September 18, 2001.REUTERS/NYC Office of Emergency Management

That number includes 343 firefighters and paramedics and 60 police officers who rushed to help in the aftermath.

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Another 40 people were killed in Pennsylvania and 184 died in DC, for a total of nearly 3,000 people.

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Rescue efforts at Ground Zero continued until Oct. 9, and the flames from the aftermath of the collapse burned until December 20.

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In the months after 9/11, the nation came together to help those affected by the attacks. Blood banks were overwhelmed with donations and hundreds of people volunteered to sift through rubble at Ground Zero.

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After the terrorist attacks, President Bush declared a "War on Terror," targeting the Al Qaeda terrorists responsible for the attack and killing their leader, Osama Bin Laden, ten years later. The response included a large expansion of America's security efforts.

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This is the skyline today, with the Tribute in Light marking the spot where the towers once were.

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Now, the 9/11 Memorial is open to the public to commemorate the tragedy.